natural disasters final exam review.docx
natural disasters final exam review.docx

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University of Toronto St. George
Earth Sciences
Dan Dolderman

Natural Disasters Exam Review Meteorites Impacts - no human in the past 1000 years has been killed by a meteorite - in 1954, a 5-kg meteorite crashed through a house in Alabama the object bounced off a radio and hit the owner in the head - it is very rare for people to experience a meteorite impact there are not many impact sites Indirect Evidence of Meteorite Impacts - preserved craters on the continent (mainly the oldest parts) - Lac cratere in northern Quebec is a simple crater its diameter in 3.4km and it is 250 meters deep o The crater is is Northern Quebec the rim indicates that it is a youthful site because over time the rim is usually washed away and eroded Manicouagan - the Manicouagan crater in Quebec is a spectacular example of a complex crater - its original rim has been removed by erosion the current diameter is 100km - it has an uplifted central core and outer rings that are filled by a lake - its age coincides approximately with a large extinction at the end of the Triassic period Definitions - meteoroid: matter revolving around the Sun or any object in planetary space too small to be called an asteroid or a comet o usually most meteoroids head towards the surface but burn up before they reach the surface of the Earth - meteorite: a meteoroid which reaches the surface of the earth without being vaporized o meteorites come from larger parent bodies within our solar system Asteroids - rocky fragments that either fail to consolidate into a planet or represent remnants of a fragmented planet Asteroids and the Asteroid Belt - the Asteroid Belt lies between Mars and Jupiter there are about 4000 objects in this area as asteroids collide with one another, they fragment and send pieces into near-Earth orbits - - Types of Meteorites Derived from Asteroids - asteroids have a metallic core and stony silicate mantle as the fragment, both metallic and silicate pieces are produced Stony Meteorites - 94% of all meteorites - two types: o chrondites: contain chondrules very old and primitive o achondrites: no chondrules Iron Meteorites - these consist of nearly pure metallic nickel and iron Stony-Iron Meteorites - these are a mixture of the two previous types - they are often fragmental suggestive of violent processes o indicates a violent origin of one body hitting another - these are very rare Comets - comets come from the far reaches of the Solar System - they have highly elongated, elliptical orbits which bring them close to the Sun - they mainly consist of ice and dust, thus are referred to as dirty icebergs or dirty snowballs - they are held together very loosely Impact Events 1. Probabilities of A Collision a. What are the chances of a large meteorite hitting Earth? i. As of 2008, 757 objects with diameters larger than 1km are know to have orbits which intersect that of Earth 30 new objects are discovered each year, with only 8% of the search complete ii. Zebrowski estimates that, on averahe, collisions of 1km-diameter objects occur every 250 000 years an impact of this size would wipe out most of the human population iii. Courtillot expects it is about 1 Ma between events iv. The recurrence intervals are very rare there is a low probability for a very high impact event (is it worth spending money to research this if there is little chance of it happening?) v. Things that have a diameters of greater than 1km are believed to have global effects if they hit the Earth b. Regardless, meteorite events are very rare and very destructive 2. Nature of the Event a. Impact cratering in an important process in the history of the Earth and other planets b. 10^7 10^9 kg of meteoritic flux strikes Earth each year, mostly in the form of dust c. the cratering process is very rapid since the objects travel so fast (4- 40km/second) a huge amount of energy is transferred upon impact d. Cratering: i. The main elements are high pressure and high temperature ii. A blanket of ejecta is dispersed around the crater rock is fractured, crushed and broken in large impact event, the rock can even be vaporized iii. Very high pressures are reached, resulting in shock metamorphism (pressure and temperature increases) iv. After initial compression comes decompression, which may cause the rock to melt e. Simple Craters: i. They are basically simple bowls, with time the ejecta blanket outside the crater is eroded ii. There are large rocks near the rim the bedrock is fragmented because of the impact event (the degree of fracturing decreases the further down the bedrock you go) iii. The size of the crater is much larger than the size of the meteorite f. Complex Craters i. They re generally generated by the rebound of the central core this core, as it decompresses, may melt ii. The central uplift is due to the decompression after the event has occurred iii. Sometimes things are decompressed so rapidly that the rock melts 1. Looks like frozen liquid that was once solid rock 2. It instantly liquefies and then has been cooled (glass) g. There are about 200 large, well-preserved impact craters world-wide (there are older impact sites that are buried) h. Cratering on the Moon i. There is a big difference between the amount of craters on Earth and on the moon i. Why are there so few impact craters on the Earths surface? i. In many cases there have been impacts in the geological past and since have been covered by sedimentary rock ii. Erosion is more prominent on the Earth and more so than any other planets iii. Continental collision destroyed evidence of impacts iv. Evidence of impacts are lost when oceanic plates get subducted 3. Consequences a. These would apply for an object of 1km or larger b. Consequence 1: a base surge, similar to a volcanic pyroclastic flow, will be generated by the impact for terrestrial impact, rock will be pulverized or vaporized, sending a huge amount of dust into the stratosphere c. Consequence 2: for oceanic impact, huge amounts of water will be vaporized because of the large amount of heat, energy is liberated upon impact because they are travelling so fast, runaway hurricanes called hypercanes may be formed, global tsunamis will be generated that will ravage the Earths coastlines d. Consequence 3: in the short term, global wildfires will be generated by the impact, these fires will burn uncontrollably across the globe, sending more soot, dust and gas into the atratosphere e. Consequence 4: all this suspended dust and soot will cause global winter and global darkness (reflect back sunlight, stop sunlight from reaching the earth), acid rain will fall, crops will fail, the end result will be mass extinction f. Consequence 5: the impact will likely trigger devastating quakes around the globe, especially where tectonic stresses are high volcanism may occur on the opposite side on the globe from the impact, as a result of shock waves travelling through the center of the Earth 4. Mitigation a. The problem is the possibility of little or no warning there are proposals to use nuclear weapons and satellites to shoot down or destroy such killer objects also difficult due to size of meteorites Three Case Studies 1. Tunguska 1908, Russia a. June 30, 1908 something big seemed to have exploded in the atmosphere the exact cause is uncertain, but a comet or meteor is suspected b. The objects entry appeared to be at an angle of 30-35 degrees the object shattered in a series of explosions at about 8km altitude c. In the central region, forests flashed to fires which burned for weeks d. Mysterious event it occurred in a sparsely populated area, which is why there is such poor documentation of the event e. A herd of 600-700 reindeer were incinerated f. About 2000 km squared were flattened by the blasts g. The best scientific guess is that it was part of a comet 20-60 meters in diameter, no crater was found and no meteoritic debris was found h. The lack of a crater suggests disintegration above the surface of the Earth the lack of solid debris implies a comet rather than an asteroid i. Soot from the fires circled the globe, producing spectacular sunrises and sunsets for months afterward j. It is the largest kn
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